In the upcoming age of nanotechnology, where devices may be a thousand times smaller than the microchips of today, semiconductor nanocrystals (Quantum Dots) have exhibited charge storage capability. This promises to be useful in future non-volatile memory applications and is currently under investigation by various research groups.
Charge storage devices that exceed the performance limits of a conventional floating-gate device have attracted a great deal of interest and are spurring rapid progress in this area. Quasi-nonvolatile MOS memory devices employing silicon nanocrystal charge-storage sites produced by ion implantation into the gate oxide have already been demonstrated (Tiwari et al., Appl. Phys. Lett. 68 (10), p.1377, 1996) or germanium (King et al. IEDM Tech. Dig., 1998, p. 115). Devices with embedded silicon or germanium nanocrystals, fabricated using ion implantation, exhibited superior data-retention characteristics as compared with conventional floating-gate devices. However, the ion implantation technique has its limitations such as a long processing time, a non-uniform germanium profile in the oxide and a compromise on the control oxide and interface quality. In addition, ion implantation places a lower limit on the top control-oxide thickness.
It has been suggested to use radio-frequency co-sputtering and rapid thermal annealing to form the oxide layer containing germanium nanocrystals (docket number CS01-074, Ser. No. 10/087,506, file date Mar. 1, 2002, now U.S. Pat. No. 6,656,792, assigned to a common assignee as the instant invention). A high quality layer of thin tunnel oxide is first grown by rapid thermal oxidation prior to the sputtering process. However, the subsequent sputtering step results in a non-stoichiometric oxide film and also has a problem with particle generation.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,128,243 Chan et al. show a memory for a SRAM using germanium Nanocrystals. U.S. Pat. No. 5,783,498 (Dotta) shows a process to form germanium Nanocrystals. Sugiyama et al. show a memory device using germanium Nanocrystals in U.S. Pat. No. 6,060,743 while, in U.S. Pat. No. 6,090,666, Ueda et al. show another memory device using germanium Nanocrystals.